Impact pile driving in water produces extremely high sound levels in the surrounding environment in air and underwater. For example, underwater sound levels as high as 220 dB re 1 μPa are not uncommon ten meters away from a steel pile as it is driven into the sediment with an impact hammer. Vibratory pile drivers, sometimes referred to as vibratory hammers, are also known in the art. Vibratory installation of piles produces lower peak acoustic pressure in the water but total energy of the radiated noise is still very high. A conventional vibratory pile driver is clamped onto the upper end of a pile that is to be driven into the sediment, and may be positioned and supported over the pile with a crane or the like. The vibratory pile driver generates relatively high frequency vertical impulses to the pile, which loosens or liquefies the sediment that engages the distal end of the pile, such that the weight of the pile (and the vibratory driver) urge the pile into the sediment.
Reported impacts on wildlife around a construction site include fish mortality associated with barotrauma, hearing impacts in both fish and marine mammals, and bird habitat disturbance. Pile driving in water is therefore a highly regulated construction process and can only be undertaken at certain time periods during the year. The regulations are now strict enough that they can severely delay or prevent major construction projects.
There is thus significant interest in reducing underwater noise from pile driving either by attenuating the radiated noise or by decreasing noise radiation from the pile. As a first step in this process it is necessary to understand the dynamics of the pile and the coupling with the water as the pile is driven into sediment. The process is a highly transient one in that every strike of the pile driving hammer on the pile causes the propagation of deformation waves down the pile. To gain an understanding of the sound generating mechanism the present inventors have conducted a detailed transient wave propagation analysis of a submerged pile using finite element techniques. The conclusions drawn from the simulation are largely verified by a comparison with measured data obtained during a full scale pile driving test carried out by the University of Washington, the Washington State Dept. of Transportation, and Washington State Ferries at the Vashon Island ferry terminal in November 2009.
Prior art efforts to mitigate the propagation of high sound pressure levels in water from pile driving have included the installation of sound abatement structures in the water surrounding the piles. For example, in Underwater Sound Levels Associated With Pile Driving During the Anacortes Ferry Terminal Dolphin Replacement Project, Tim Sexton, Underwater Noise Technical Report, Apr. 9, 2007 (“Sexton”), a test of sound abatement using bubble curtains to surround the pile during installation is discussed. A bubble curtain is a system that produced bubbles in a deliberate arrangement in water. For example, a hoop-shaped perforated tube may be provided on the sediment surrounding the pile, and provided with a pressurized air source, to release air bubbles near or at the sediment surface to produce a rising sheet of bubbles that act as a barrier in the water. Although significant sound level reductions were achieved, the pile driving operation still produced high sound levels.
Another method for mitigating noise levels from pile driving is described in a master's thesis by D. Zhou titled Investigation of the Performance of a Method to Reduce Pile Driving Generated Underwater Noise (University of Washington, 2009). Zhou describes and models a noise mitigation apparatus dubbed Temporary Noise Attenuation Pile (TNAP) wherein a steel pipe is placed about a pile before driving the pile into place. The TNAP is hollow-walled and extends from the sediment to above the water surface. In a particular apparatus disclosed in Zhou the TNAP pipe is placed about a pile having a 36-inch outside diameter. The TNAP pipe has an inner wall with a 48-inch O.D., and an outer wall with a 54-inch O.D. A 2-inch annular air gap separates the inner wall from the outer wall.
Although the TNAP did reduce the sound levels transmitted through the water, not all criteria for noise reduction were achieved.